How about letting people try their preferred schedules, and then circling back and showing (some) people that it's in their interest to follow a "9-5" schedule

How about letting people try their preferred schedules, and then circling back and showing (some) people that it's in their interest to follow a "9-5" schedule because you actually saw them performing better? And then leave it to them to decide if they want to underperform and face consequences, or do what works well? At my job I get paid how good my boss thinks my work is, not how good he thinkgs it could be if I worked differently.